A) Prior Invention
In the prior invention (application Ser. No. 317,767), an industrial computerized tomographic system is disclosed which has as one of its principal features, when compared to other systems, a very fast scan time. One aspect of the prior invention which is significant with respect to the fast scan time, is the use of a three dimensional cone shaped beam of hard radiation to irradiate the scanned object. A truncated attenuated beam of radiation then impinges upon a two dimensional detector arrangement to produce data which is digitized and used in the computer to reconstruct images of the object. The present invention relates to an arrangement or a system which converts the energy in the attenuated radiation beams into digitized data which is then subsequently used to reconstruct three dimensional object images, etc. (hereinafter referred to as "imagers" or "imaging devices").
B) Prior Applications
Image intensifiers applied to x-ray systems are well known. Typical devices are shown in Wang U.S. Pat. Nos. 4,255,666 and 4,426,721. The Wang devices include a sealed housing constructed so as to draw a vacuum which have a metal input window through which attenuated beams of radiation pass. The attenuated x-ray beams, after passing through the metal window, strike a scintillator/photocathode screen and are converted into photoelectrons and, through the electrical potential developed within the intensifier, accelerated, so as to strike an output phosphor display screen. The display screen produces an intensified light image corresponding to the irradiated object. Typically, the image produced in the output plate is viewed by means of a TV camera such as shown in Haendle U.S. Pat. No. 4,658,410 or focused by mirrors into a video process such as shown in Beekmans U.S. Pat. No. 4,809,309. Besides the use of television cameras, such as vidicons and plumbicons, Wang '721 shows that the light from the output window of the intensifier can be directed to photodiodes which function as detectors producing an analog signal which in turn is digitized, etc. Also, Green U.S. Pat. No. 3,758,723 uses an orthicon camera tube in place of the output display screen and the electrons from the photocathode (i.e. light intensifier) are focused by an electromagnetic lens to produce an electron image on a target which is recorded by the orthicon camera tube. All of the arrangements discussed (with the exception of Wang '721) appear to be directed to arrangements which produce two dimension radiographs.
It is known to use charge-coupled devices (hereinafter CCD) in place of the video tubes noted above. For example, Catchpole U.S. Pat. No. 4,675,739 discusses the benefits of CCD devices but notes that such devices are not suited for large area arrays of image sensors. To a similar effect is Goldman U.S. Pat. No. 4,298,800 which discloses a CT application for a fan shaped beam of attenuated radiation which is converted to visible light by a tilted fluorescent screen or a scintillation crystal. The light passes through a lens arrangement which focuses the imaged object directly onto a CCD device. When several CCD devices are used, several different lens systems are employed. While the arrangement disclosed may be acceptable for small fan shaped areas in the medical imaging environment, the multiple lens system employed in Goldman is impractical for industrial application and it is doubtful that signal strength will be sufficient to permit imaging in an industrial environment.
A focusing lens system is also utilized in Green '723 where attenuated x-ray beams pass through a fluorescent screen and are reflected by a mirror orientated at approximately 45.degree. through a lens system which directs the image onto a convex lens photocathode to develop two dimensional radiographs. In Green, the workpiece is intermittently rotated so that the orthigon camera can integrate the image produced while the workpiece is stationary and the time for developing the image is unacceptably slow for systems to which this invention relates. Green is limited to two dimensional radiographs with a field of view of about six inches in maximum dimension at a processing time of about 1 second per view and with a resolution of objects within the view of about 1/30" in measurement. Thus, in Green the size of the field of view, the time for developing the image and the sharpness or picture resolution of the developed image is unacceptable for industrial applications under discussion.